1 Preface
Sheet metal bending is to use the general mold (or special mold) equipped with the CNC bender to bend the sheet metal into parts with various required geometric section shapes.[1]. The rationality of the folding process directly affects the final formed size and appearance of the product. Reasonable selection of bending molds is crucial for the final shaping of the product.
In the actual production process, due to the uncertainty of product size and the diversification of product types, we often encounter problems such as dimensional interference and mismatch of mold angles when bending processed parts cold, which causes great production difficulties.[2]. Since the bending process is affected by factors such as product size, shape, materials, molds, equipment and auxiliary facilities, various quality problems will arise, affecting production efficiency and stability of product quality. Therefore, it is particularly important to know how to resolve and prevent the occurrence of these quality problems. This article mainly summarizes and describes common sheet metal bending quality problems in production practices, analyzes the causes, and puts forward solutions based on production experience.
2 Common Fold Quality Problems
2.1 Bending and cracking
Flexural cracking refers to the phenomenon that burrs or small cracks often appear on the edge of the material after cutting, shearing or stamping. When folding, it is easy to form stress concentration and break. ) bending The cracking situation at the rear corner is shown in Figure 1.
Figure 1 Flexural cracking
The main causes of bending cracking are: ① Burrs on the edge of the workpiece are not eliminated. ②The folding direction is parallel to the rolling direction of the plate. ③The radius of curvature of the sheet is too small.
During the manufacturing process, bending cracks need to be treated according to specific conditions. Considering the flexural cracking problem in Figure 1, it can be solved by adding process holes or process slots, as shown in Figure 2.
Figure 2 Adding Process Holes
2.2 Bending interference
Bending interference mainly affects products folded twice or more. The bending edge collides with the mold or equipment, preventing the product from forming normally. Bending interference is mainly affected by the shape, size and mold of the part. It is mainly caused by the design structure of the bending part itself, the selected bending sequence and the optional bending mold.[3]. Therefore, the main solutions are: ① Make new molds or replace molds (such as machete molds). ② Modify the bending mold (such as mechanical processing of parts). ③Adjust the folding sequence (such as anti-deformation method). ④Change the bend size of the part. For example, Shanghai Line 18 chassis accessory gutter installation bracket (ADC1027252G030), the accessory is made of U-shaped channel steel, the center width is 100mm, the side height is 80mm and the radius of curvature is 15 mm. Simulate bending from existing molds in the shop to generate bend interference.
In order to solve this interference phenomenon, the partial machining method of the bending upper mold is used (see Figure 3), and a space of 140mm × 48mm is cut out in the center line of the R15 straight knife upper mold mm existing (L = 800 mm). ) (see Figure 4). The position of the notch is determined based on the position of the simulated bending interference, without affecting its original function. After modifying the bending mold, the bending interference problem was successfully solved.
Figure 3 Bending after upper mold processing
a) Bending interference b) Determine the treatment area
Figure 4 Bending interference, determination of treatment area
2.3 Folding and indentation
Bending indentation is a phenomenon in which the friction force generated during the close and gradual contact between the sheet metal and the inner surface of the V-shaped groove of the die leads to obvious marks on the surface of the sheet material . For some accessories with high area requirements, traditional bending cannot meet the product quality requirements, and bending indentation (see Figure 5) cannot meet the requirements of the following process.
Figure 5 Bend Indentation
Bending indentation is mainly affected by the hardness of the plate and the structure of the bottom mold. The greater the hardness of the plate, the greater its ability to resist plastic deformation, the more difficult it is for the material to produce plastic deformation, and the easier it is to produce indentations. The probability of bending indentations for commonly used plate materials is. : aluminum > carbon steel > stainless steel[4]. The larger the opening width of the lower bending die, the larger the width of the bending indentation and the shallower the depth of the indentation. The larger the size R of the lower shoulder of the die opening, the smaller the indentation depth.
In addition to improving the hardness of the material and the bottom structure of the mold to solve the problem of bending and indentation, anti-indentation rubber pads and ball-type bottom bending methods can also be used. Anti-indentation rubber pads primarily rely on physical insulation to reduce the occurrence of indentations, as shown in Figure 6. The lower ball bend die converts the extrusion friction required for traditional bend forming into rolling friction, thereby reducing friction and causing almost no damage to the product by the die, as shown in Figure 7.
Figure 6 Anti-indentation rubber pad
Figure 7 Lower ball bending die
2.4 Elastic return in flexion
During the process of bending materials, both plastic deformation and elastic deformation occur. When the workpiece leaves the bending die, elastic recovery occurs, making the shape and size of the bent part inconsistent with the load. This phenomenon is called elastic return in bending.[5]. Springback in flexion is one of the main reasons why the flexion angle is out of place. The factors that affect springback are mainly the mechanical properties of the sheet metal and the bending deformation conditions. The rebound value is directly proportional to the elastic limit of the sheet and inversely proportional to the elastic modulus. The smaller the relative radius of curvature of the bent part (the ratio between the radius of curvature and the thickness of the sheet R/t), the smaller the value of the elastic return to bending. The shape of the bent part also affects the bending rebound value. Generally, U-shaped parts have a lower rebound value than V-shaped parts.
The main method to overcome springback in bending is the angle compensation method. Generally, a slope equal to the springback angle is adopted on the bending mold, which can effectively balance the impact of springback in bending. As shown in Figure 8, using a bending die with a slope of 80°, a workpiece with a bending angle of 90° can be bent smoothly.
Figure 8 Compensation for springback in bending
Since many factors affect the springback in bending, it is very difficult to accurately calculate the springback value. Through mold trial correction and experience accumulation, it is an effective method to ensure product quality by mastering the law of springback, taking appropriate compensation and taking special measures to overcome springback in mold structure and other aspects.
2.5 Bending and sliding of the material
Bending slippage refers to the phenomenon that the workpiece to be bent does not have complete and effective support points on the bottom groove of the die, causing the workpiece to slide easily and the bending cannot be positioned correctly.
The main reasons for flexural slippage are as follows.
1) The width of the bottom bend die is too large, and when the bend size is less than half of the width of the bottom die, material slippage occurs.
2) The workpiece is affected by the shape and size, and when the positioning size of the model is too short or there is no effective positioning edge of the model, bending and sliding will occur .
There are two main ways to solve the bending slip problem.
1) Method 1. Choose a suitable bending die. Generally, choose a die width of 4 to 6 times the plate thickness for bending.
2) Method 2. By adding jigs or process edges, the problem of material slippage caused by failure to properly position elbows is solved. Generally speaking, the bend is positioned with a straight edge of the part, which requires two end faces to be in contact with the bend jig. However, in the actual production process, there are cases where the edge of the product model is too short or is too short. no effective positioning edge, which prevents folding positioning from being completed. The solutions are as follows: ① When the plate thickness t≤6mm, add a process edge for positioning. The protruding position of the process edge is flush with the end edge of the accessory. The intersection is cut with a laser slot for easy grinding and removal. once the folding operation is completed. ② When the thickness of the plate is >6mm, the template can be cut for positioning. The thickness of the template can be equal to or slightly less than the thickness of the part. As shown in Figure 9, both positioning methods can solve the problem of material bending and sliding.
Figure 9 Adding process edges or templates
2.6 Large arc bending
During the manufacturing process, it often happens that the part has a large curvature radius and the workshop does not have a suitable large arc mold. In this case, the production cycle of integrally formed pressing molds or large arc molds is long and the cost is too high. However, using a small arc multi-pass bend forming process has lower costs and wider applicability. For example, the 3 to 1 position vertical plate, accessory of the Super Bus 2.0 project (ADC1043361G030), has a curvature radius of 125 mm and a curvature angle of 90°, as shown in Figure 10. The workshop does not have no corresponding bending molds, multi-pass bending processes can be used. First, use 3D software to model the loft and bend at the R125mm position. After modeling, use the software to automatically enlarge the flat 2D drawing by inputting the curvature radius of 45mm into the software and making several data input comparisons, confirm. that the folding is successful. The 8-knife formation can guarantee arc segments, and then generate the bending data (bending angle, bend line position length) of each knife, as shown in Figure 11. Finally, bending test On-site mold design is carried out based on the bending data, as shown in Figure 12.
Figure 10 Large arc part
Figure 11 Enlarged view and position of the fold line
Figure 12 On-site mold bending test
2.7 Bending bulge
Bending bulge is a phenomenon in which the metal material protrudes due to the extrusion of the material on both sides of the sheet metal corner after bending, making the width larger than the original size. The size of the curvature projection is generally related to the thickness of the accessory plate and the radius of curvature. The greater the thickness of the plate, the smaller the radius of curvature and the more obvious the protrusion.
In order to avoid this problem, you can add process spaces on both sides of the bend line when drawing the bend expansion diagram, as shown in Figure 13. The process space is usually below the shape of an arc, and the diameter of the arc is generally greater than 1.5 times the thickness of the workpiece, thereby compensating for the curvature bulge and effectively solving the problem of curvature bulges. For parts with curvature bulges, they are usually processed by manual grinding.
Figure 13 Process Gap
3 Conclusion
It should be noted that the common folding and cutting quality issues listed above in production practices do not take into account the impact of human or material factors (such as poor expansion dimensions, poor selection folding parameters, aging equipment, etc.). In production practice, the appropriate parameters of the bending process should be selected according to the performance of the equipment, product size and material properties, and strictly implemented in accordance with the operating specifications. We not only need to comprehensively consider the impact of various factors such as project progress, cost and quality, and adopt appropriate methods to solve bending quality problems, but also predict the occurrence of problems of folding during the analysis phase of the process through the accumulation of experience and. impact and take targeted measures to avoid it. This article lists several common problems and solutions related to bending quality, hoping to provide a reference for colleagues in the industry.
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